Many activities are undertaken under the umbrella of manufacturing.
Each of these activities affects the surface finisher in some
way. Most materials can be used in manufacturing including
metals, non-metals and organic materials.

There is a large list of common manufacturing activities
and these will be discussed in groups as follows:

Molten Activities

Casting

Forming/shaping activities:

Stretch forming

Peen forming

Deep drawing

Spinning

Rolling activities

Roll forming

Thread rolling

Roll bonding

Joining activities

Welding

Soldering

Brazing

Adhesive bonding

Diffusion bonding

Cutting activities

Machining

Punching

Grinding

Powder metals

Plastics:

Thermoforming

Rotational moulding

Blow moulding

Moulding

Extrusion

All these activities affect the surface of the material as
well as the metallurgy. Surface finishes are, as the name
implies, finishes that are applied to the surface of metals
and materials so any activity that affects the surface of
the material to be coated is of interest to surface finishers.

Pressure shaping activities

Coining

Forging

Extrusion

Wire drawing

Heading

History

Before 4000BC: Pressure shaping activities were the
first ones used by man. Pure metals such as gold, copper
and meteoritic iron were hammered into shapes.

4000-3000BC: Jewellery was produced by stamping activities.

3000-2000BC: Wire was produced by cutting sheet and
drawing metal through dies.

1000-1BC: Coin stamping.

AD1-1000: Steel developed, and coining, forging and
the production of steel swords.

All these activities affects the surface of the material
and hence the coating.

What is it we are trying to coat?

Is it the surface the customer has sent us? Or is it the
surface below what the customer has sent? The exact composition
of the surface to be coated must be known if a quality coating
is to be applied. It is necessary to ensure the correct pretreatment
is applied.

The surface that the customer sends you may not be the metal
surface to be coated; eg bright stainless steel is often coated
with a clear strippable coating to protect the finish. Also,
some steel tube is manufactured and marketed with a thin organic
coating applied.

Metals can be identified mainly by weight, colour and magnetism.

Metal

Specific Gravity

Magnesium

1.74

Aluminium

2.7

Zinc

7.0

Iron

7.87

Copper

8.96

Silver

10.5

Gold

19.32

Specific Gravity
of some metals

The Metals

Aluminium

Aluminium is a very light, malleable silvery metallic
material that is resistant to many forms of corrosion.
Halides eg chlorine found in salt water will attack aluminium.

The metal is non-magnetic.

It is sometimes difficult to distinguish some anodised
aluminium surfaces from mill finish. Anodised surfaces
are harder and more abrasion resistant than mill finished
aluminium surfaces. A drop of caustic soda or nitric acid
will react very quickly with a degreased mill finished
surface whereas the attack will be very much slower on
an anodised surface.

Anodised surfaces should not be powder coated. Remove
the oxide by etching if powder coating is required.

Aluminium Castings

Molten aluminium is poured into a mould and allowed
to solidify to take the shape of the form.

Some die cast and sand cast alloys, particularly have
high silicon contents and smut will form very easily during
pretreatment operations.

Die casting is usually done under pressure whereas in
an open mould casting eg sand casting the metal is simply
poured into the mould at atmospheric pressure. The die
castings (pressure injected) are less porous than the
open mould castings.

Aluminium and Zinc Spray

In metallising a metal is sprayed while hot onto the
surface to be protected. The surface of a metallised surface
eg aluminium or zinc is relatively porous and looks a
little rough and white.

Cold Rolled Steel

The most common metal to be coated with powder is steel
(iron), in one form or another. Iron and all mild steels
are magnetic. The colour of the surface can vary depending
on the condition eg rust, mill scale, abrasive blasted,
bead blasted, etc. It can come into the shop coated with
paint, grease, oils, etc.

Hot Rolled Steel

The surface of hot rolled steel will be covered by mill
scale in various stages of adhesion. When continuous the
scale is a blue black colour, but often it has been damaged
and rust, etc will protrude through the mill scale. All
mill scale and other oxides must be removed before a coating
is applied.

Cast Iron

Cast irons are generally magnetic - (although some of
the cast steels may not be magnetic). Cast irons contain
high levels of carbon, which can provide cleaning problems.
Probably best to abrasive blast clean prior to phosphating.
The surface of cast irons can be porous and may require
the use of special powders for a good quality coating.

Copper

Copper is a reddish brown metal, which is non magnetic.
It conducts heat and electricity very well. It is sometimes
used for decoration.

Brass

Brass is a non magnetic yellow metal - an alloy of copper
and zinc. It is usually malleable and in the highly polished
state 70/30 brass resembles gold although it tarnishes
very quickly. Clear coatings are applied to reduce this
effect. The higher the copper content the more difficult
to maintain a good gold colour when applying a clear finish
to the metal.

Lead

Lead is a silvery white, soft, malleable metal used
for piping and in numerous alloys. Lead is extremely heavy
and non-magnetic. Lead is the main constituent in solders
which have a low melting point. This can pose problems
(such as disassembly) when soldered parts are to be powder
coated.

Zinc

Zinc is a bluish white, non magnetic metal. Zinc may
arrive in the form of a die casting, a coating on steel
(galvanize or electrogalvanize). As a coating the colour
can vary from gold, green, yellow to blue/white to black.
The metal is also used in zincalume, zincanneal coatings
on steel sheet components.

Stainless Steel

Stainless steel always contains iron and chromium and
may also contain appreciable amounts of nickel. Depending
on the composition and the condition of the metal it may
be magnetic or non magnetic. Much of the stainless steel
that comes to a powder coater is highly polished and coated
with a protective material.

The Generation of Shapes

Metal products may be produced either by casting or by
working metals. We hear the terms cast metals and wrought
metals.

Casting

Metal is heated until it melts and is then poured into
a mould.

A mould is created of the shape to be produced. This
mould may be made from a metal or sand.

Metal moulds produce castings with good finishes.

Sand moulds tend to produce rougher castings. Sand moulds
are composed of clean silica sand, and organic materials
which help to bind the sand together to provide the strength
to resist fracture during the casting operation. The sand
is rammed into the desired shape of the part to be cast.

When molten metal is poured into a sand mould gases
are produced when the organic material burns. These gases
must escape and the routes available to them are:

through the metal which is solidifying, or

through the sand.

If the gases escape through the metal, porosity will be
formed in the part. These pores contain gases and pin holes
will be formed in the cured powder coating if they are not
removed.

If the gases are to pass through the sand, the compactness
of the sand has to be reduced so that the gases can pass
though. As the sand is compacted less and less it loses
strength and may crumble when the molten metal is poured
into the mould. If this occurs, sand will be entrapped in
the surface of the part that is cast and this will cause
roughness in the powder coating, or bad adhesion, or both.

A parting compound is applied to metal moulds to assist
the removal of the casting. This material must be free of
silicones if the casting is to be coated.

Wrought Metals

Wrought metals come in many shapes and sizes.

Extrusions

This process was first developed in the late 1700's
for producing lead pipe.

Metal is cast into billets, which are homogenised so
that the constituents (parts) of the metal are evenly
distributed) and the billet forced through a die whose
shape is the final shape required in the extrusion. The
extrusion process may be carried out hot or cold. After
it is extruded the workpiece is then heat treated to remove
stresses and to achieve the required mechanical properties.
Finally the extrusion is straightened and cut to length.

There are four basic types of extrusion:

Direct

Indirect

Hydrostatic

Impact

The surface of extrusion as received from the mill is
very heavily oxidised. Surface cracking can occur in aluminium,
magnesium and zinc alloys.

Sheet and Strip

Sheet is produced by rolling a billet of metal repeatability
until the desired thickness is obtained. Between rolling
operations the metal is annealed to re-soften it so
that more rolling can be carried out. Rolling was first
developed in the late 1500's. It is the process of reducing
the thickness or changing the cross section of a workpiece
by compressive forces exerted by a pair of rotating
rolls.

The surface of rolled sheet can be provided as:

Hot rolled mill finish

Cold rolled, or

Bright finish

Hot rolled material is heavily oxidised. The oxides on
cold rolled and bright material are thinner.

Hollow Sections

Hollow sections can be extruded or produced by roll
forming strip to the desired shape and resistance butt
welding the seam. To improve the external appearance
of the tube the external weld is usually cleaned off
mechanically.

Extruded sections are heavily oxidised as is the weld
on butt seam welded tube. Rolled tube surfaces may contain
non-adherent metal chips from the rolling operation.

Fabrication

Metals can be fabricated using a vast array of techniques
for cutting, sawing, milling, machining, bending, rolling,
drawing, forming, punching, blanking and spinning.

Metals can be joined together to produce a fabricated
part by mechanical, chemical (adhesive) and metallurgical
processes.

Bending

Metals are bent using conventional presses, folding
machines, tangent benders, lock seamers, roll formers
and draw and stretch benders. To bend hollow sections
such as round, square or rectangular tubes, mandrel
or draw bending is recommended. For less onerous applications
wrap or compression bending, crush bending or roll
bending can be used. When metal is bent, the outer
fibres of the material are in tension and the inner
fibres in compression.

Bending operations can leave 'spalled' semi-attached
metal particles on the surface of the metal as well
as any lubricants that may have been used to assist
the process.

Shape Rolling

Shape rolling was first developed in the late 1700's.
Various shapes such as bars of various cross-sections,
various open sections and railway tracks can be produced
by passing metal through a number of pairs of specially
designed rollers.

The surface may be contaminated with rolling lubricants,
and/or non-adherent metal particles in the form of
scoring residues.

Deep Drawing/Forming

Deep drawing was first developed in the 1700's. In
forming, the metal is shaped into a component within
a die. In deep drawing, the metal is ironed and stretched
or compressed during the operation.

The surface can be contaminated with drawing lubricants
that generally are very adhesive and often chemically
bonded to the steel surface. Non adherent metal particles
in the form of scoring residues are common. These are
more likely to appear on highly oxidised surfaces such
as aluminium, etc.

In rod and wire drawing, the cross-sectional area of
a bar is reduced by pulling it through a converging
die - a little like the reverse of extrusion.

Shearing, Punching and blanking

When punching and blanking two processes take place
- the metal is cut and then sheared. If you look at
a punched or blanked cut you will see a shiny part (the
cut) and a rough grainy part (the sheared part). If
the tools are correctly designed and maintained the
cut portion is about two thirds to three quarters of
the edge metal thickness. When punching or blanking
pre-galvanized sheet, the zinc on the surface is wiped
over the cut portion and provides some corrosion resistance
to the edge.

The rough, grainy portion of the edge gives rise to
Faraday Cage effects (in powder coating and highly varying
current density effects in electroplating) that producing
a specification coating is difficult at best and often
impossible.

Spinning

A rotating metal blank is pressed against a back up
chuck of the desired shape. It is often used for spinning
aluminium and aluminium alloys and 70/30 brass into
parabolic, spherical, conical, tapered or re-entrant
shapes. It is an economical process for small quantities.

The surface of the metal is highly worked and some
lubricant may be present. The surface is relatively
easy to prepare for coating.

Cutting, sawing, drilling, machining and turning

Lathes were first developed in the 1800's. All these
operations may be termed material removal processes.

In these operations the machined surface should be free
of defects and may contain oils. However, if the tools
are not designed correctly nor properly maintained, then
it is possible to produce burrs and rough surfaces, which
can affect the coating process.

Cutting oils should be free of silicones. All silicones
are difficult to remove and interfere with all coating
processes.

Powder Metals

Metal powders are compressed in a die and subsequently
sintered by heating at elevated temperatures. The process
was first developed in the 1840's. Powder metal components
are not usually coated.

Joining

Metals may be joined by mechanical, chemical or metallurgical
techniques.

Mechanical

Mechanical techniques include riveting, threaded fasteners,
stapling are used extensively to join metals.

All these joining processes produce spaces between
the joined metals and between the fastener and the metal
being joined. Liquids will ooze out of these loose joints
causing problems with all forms of coating processes,
including staining and non-coverage. Most coating processes
require the space between the sections to be at least
as wide as the depth of the space, eg to effectively
coat the bottom of a hole, the diameter of the hole
should be at least the same dimension as the depth of
the hole

Adhesive

Adhesive bonding using thermoplastic and thermosetting
materials is used extensively to fabricate simple and
complex assemblies. They provide sealing and insulating
properties, reducing the opportunity for electrochemical
corrosion between dissimilar metals. Adhesives can also
reduce vibration through internal damping at the joints.

The problems for a coater are similar to mechanical
joining techniques.

Heating

Heating joining techniques consist of 'mechanical'
joints (soft solders and brazing where there is no melting
of the parent materials) and welding which is a metallurgical
bond (some of the parent metal as well as any filler
material melt).

Soldering and brazing

A solder is heated and melted and is used to fill
crevices and joints.

Low melting point solders are used to fill cavities
or joints and for low strength mechanical joins.
Aggressive chemical fluxes are required to remove
the oxide film and expose the metal. These fluxes
must be removed after soldering to prevent corrosion
in service.

Low melting point solders generate a galvanic cell
with aluminium leading to corrosion. Various techniques
including ultrasonic soldering, reaction solders and
friction solders are in use to reduce this problem.

Solders contain lead. Lead is difficult to prepare
for coating because of its oxide film. Special chemicals
are used. Also, it does not easily accept a chromate
or a phosphate coating and thus is the weak link in
a powder coating situation.

Brazing solders are high temperature solders consisting
of various metal alloys depending on the metals to be
joined. For aluminium, the brazing material contains
high levels of silicon so that the galvanic corrosion
problem associated with soft solders is reduced. Manual
or torch brazing, furnace brazing and dip brazing are
used. Highly reactive fluxes are required and these
can lead to corrosion in service if not completely removed.

All solder fluxes must be completely removed or neutralised
to prevent the onset of corrosion and to ensure that
the coating adheres to the metal.

Oxyacetylene welding

A mixture of oxygen and acetylene is forced through
a torch and ignited to produce a hot flame that is
used for welding and cutting metals. For some metals
it is more difficult to use than TIG and MIG processes.
Oxyacetylene welding gives a broad heat source whereas
intense localised heating is essential for welding
metals such as aluminium. If Oxyacetylene welding
is used for these metals, preheating is required.

High levels of oxidation are produced with this
technique as well as weld spatter and slags from the
filler rods. All of these defects must be removed
before coating as they interfere with the adhesion
of the coating. In the case of weld spatter, coatings
such as powder coating pull away from the tips of
the spatter during curing leaving spatter peaks uncoated.

Metallic arc welding

Also known as stick welding, a rod is electrically
energised to produce a high temperature arc (spark)
against the metal to be welded. The 'stick' is consumed
during the process and becomes the filler metal in
the weld pool. These welds produce similar problems
for coaters, as does Oxyacetylene welding.

Resistance welding

Two pieces of metal are forced together at the same
time as a high current is passed between them. The
resistance of the metals to electricity generates
sufficient heat to melt the touching surfaces and
join them together. Resistance welds produce a weld
bead that is relatively clean except for the heavy
oxides that are produced.

TIG welding

An inert electrode (tungsten) is completely shrouded
in an inert gas such as argon, helium or a mixture
of both. A filler wire may be used. TIG provides the
conditions for high localised heat necessary for good
aluminium and stainless steel welds. When correctly
produced TIG welds should be clean with minimal oxides.

MIG welding

The Metal Inert Gas process provides heat through
an arc between the part and a consumable electrode.
As in TIG the arc zone is completely shrouded in an
inert gas such as argon, helium or a mixture. MIG
provides the conditions for the high localised heating
that is required for good aluminium and stainless
steel welds. The consumable electrode is fed into
the weld pool automatically.

When correctly produced MIG welds should be clean
with minimal oxides. Unfortunately, we see many welds
with porosity, holes, weld spatter and other defects.

Weld cracking

Cold cracking is due to the internal stresses set
up during welding if the weld is not designed correctly.
Preheating can help reduce the tendency. Hot cracking
can occur in metals and alloys that have a large liquidus
to solidus range when the metal is in a weak pasty
condition.

All cracks are difficult to coat. Fine cracks will
not be covered by electroplating and may not be covered
over with powder coating or wet organic coatings.

Porosity

Adverse conditions that give rise to hot cracking
of some alloys also encourage porosity. Porosity is
caused by gas in the weld pool not being able to escape
before the metal solidifies. All material, particularly
moisture that decomposes to form hydrogen must be
removed from the weld zone. It is also essential to
ensure the weld zone is fully shielded by the inert
gas. The natural oxide coating on the aluminium absorbs
water vapour while some of the intermetallic compounds
formed in alloys containing magnesium and silicon
react with water. Arc instability due to poor maintenance
or inefficient wire feeding in MIG welding also contributes
to porosity.

Porosity in weld is a very difficult problem for
a coater. The pores can be very fine and will 'show'
though virtually all forms of coatings.